Most of us know that Peter was the first Pope, as Christ entrusted Peter with the keys to his kingdom.
Thus in artistic images of Saints, when we see a figure of a man holding two keys, we can know that this is an image of St. Peter.

But I'll bet few of us (if any) know that the second Pope was named "Linus" ?
Here's the proof from a marble plaque in St. Peters, listing the names of popes throughout the Christian era.